x86: set CONFIG_USE_PRIVATE_LIBGCC to y

The motivation of this commit is to change CONFIG_USE_PRIVATE_LIBGCC
to a boolean macro so we can move it to Kconfig.

In the current implementation, there are two forms of syntax
for this macro:

  - CONFIG_USE_PRIVATE_LIBGCC=y
  - CONFIG_USE_PRIVATE_LIBGCC=path/to/private/libgcc

The latter is only used by x86 architecture.
With a little bit refactoring, it can be converted to the former.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index f7303ab..25b672a 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -20,9 +20,9 @@
 obj-$(CONFIG_VIDEO_VGA)	+= video.o
 obj-$(CONFIG_CMD_ZBOOT)	+= zimage.o
 
-LIBGCC := $(notdir $(NORMAL_LIBGCC))
-extra-y := $(LIBGCC)
+extra-$(CONFIG_USE_PRIVATE_LIBGCC) := lib.a
 
+NORMAL_LIBGCC = $(shell $(CC) $(PLATFORM_CPPFLAGS) -print-libgcc-file-name)
 OBJCOPYFLAGS := --prefix-symbols=__normal_
-$(obj)/$(LIBGCC): $(NORMAL_LIBGCC) FORCE
+$(obj)/lib.a: $(NORMAL_LIBGCC) FORCE
 	$(call if_changed,objcopy)